The major effort has been to determine what role immunity plays in the dissemination of Toxoplasma oocysts by the cat. Oocyst production in a primary infection results in a resistance to an oocyst producing reinfection. Cats infected first with a non-oocyst producing strain of toxoplasma are not resistant to challenge by an oocyst producing strain. Results suggest that immunization of cats using a non-oocyst producing strain of toxoplasma is not possible. Abbreviation of infection via drugs and subsequent development of immunity is being studied but results thus far are not conclusive. Comparison of the structure and biology of Toxoplasma with other coccidia, whose host specificity and pathogenicity to man are not well known, is continued. Ultrastructure of Toxoplasma and other protozoan parasites is being studied with emphasis on mechanisms of multiplication and host-parasite relationship.